Linear organo carbonate coupling agents for living polymers of conjugated dienes

ABSTRACT

New compositions and a process for the preparation of conjugated diene polymers of broadened molecular weight distribution by reacting non-terminated lithium catalyzed conjugated diene polymers with linear organic compounds selected from the group of carbonates, thiocarbonates and sulfites. The resulting new compositions are suitable for use in making high impact plastics and for fabricating rubber goods.

FIELD OF THE INVENTION

This invention relates to new compositions and to a method of preparingbranched polymers of conjugated dienes or branched block copolymers ofvinyl-substituted aromatic compounds and conjugated dienes, whichpossess one or more of the following attributes: broadened molecularweight distribution, enhanced Mooney viscosity, negligible cold flow,increased styrene solution viscosity and good processability. This isaccomplished by reacting organoalkali metal or organomagnesium catalyzednon-terminated polymers or block copolymers of conjugated dienes withlinear organic compounds selected from the group of carbonates,thiocarbonates and sulfites.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,135,716 teaches the preparation of terminally reactivepolymers through the reaction of living (i.e. non-terminated) polymerswith reagents such as oxygen, sulfur, halogen, sulfuryl chloride, carbondisulfide, carbon dioxide, and carbonyl chloride.

U.S. Pat. No. 3,598,887 teaches a process for making multiblockcopolymers by coupling living block copolymers with carbon dioxide,carbonyl sulfide, or carbon disulfide.

U.S. Pat. No. 3,281,383 teaches a method of making a branched polymer byreacting a monolithium non-terminated polymer with a compound having atleast three reactive sites capable of reacting with the carbon-lithiumbond to produce a "radial" polymer, i.e. a polymer having long chainbranches. The types of treating compounds, used included polyepoxides,polyisocyanates, polyimines, polyaldehydes, polyketones, polyanhydrides,polyesters, and polyhalides.

U.S. Pat. No. 3,349,071 teaches a process for reducing the cold flow ofdiene polymers by terminating lithium catalyzed diene polymers withcarbon disulfide.

U.S. Pat. No. 3,427,364 teaches a process for preparing polymers ofincreased molecular weight by reacting lithium catalyzed non-terminatedhomopolymers and copolymers of conjugated dienes and mono-vinyl areneswith carbon monoxide as a coupling agent.

In the Journal of Polymer Science, A-1, 6 859 (1968) there is reportedthe use of diethylcarbonate in an attempt to couple "living" lithiumpolystyrene for the formation of a ketone-containing polymers, i.e.,##STR1## which could be further used for a grafting reaction. With"living" lithium polystyrene of viscosity average molecular weight, Mv,of 31,500, and an equivalent amount of diethyl carbonate, there wasobtained a very modest increase in Mv to 42,800. However, thefractionated polymer from the diethyl carbonate reaction yielded verylittle graft polymer and therefore, it was concluded in this articlethat diethyl carbonate was ineffective as a coupling agent. The productresulting from the reaction of "living" lithium α-methylstyrene polymerof Mv=31,500 and diethyl carbonate has Mv=29,200, representing nocoupling. No mention is made of conjugated diene polymers in thisarticle, nor are any additional data or discussion given which wouldeven suggest that diethyl carbonate could function successfully as acoupling agent for broadening the molecular weight distribution of"living" lithium polydienes.

The present invention relates to new compositions and to a process formaking conjugated diene polymers having one or more of the followingfeatures: broadened molecular weight distribution, enhanced Mooneyviscosity, negligible cold flow, and better processability. In anotheraspect, it relates to a process for preparing branched copolymers havingbroadened molecular weight distribution and negligible cold flow. Duringpackaging, shipping and storage of elastomeric hydrocarbon polymers, thetendency of these materials to undergo cold flow in the unvulcanizedstate can present severe handling difficulties. If a package of polymeris punctured, the resulting polymer can flow out, leading to productloss, contamination, or sticking of packages together. Furthermore,hydrocarbon polymers of conjugated dienes of relatively high Mooneyviscosities are frequently difficult to process. Their low Mooneyviscosity counterparts on the other hand have a tendency to cold flow inthe uncured state. This restricts the use of hydrocarbon polymers ofconjugated dienes in the manufacture of high impact plastics, such aspolystyrene. Linear polybutadienes frequently do not possess thenecessary combination of rheological and viscosity properties such asMooney viscosity, styrene solution viscosity, and cold flow needed inthe manufacture of reinforced polystyrene.

We have discovered that the treatment of organolithium catalyzednon-terminated conjugated diene polymers with linear organic carbonates,linear thiocarbonates and linear sulfites produces new polymerspossessing broadened molecular weight, enhanced Mooney viscosities,negligible cold flow and greater styrene solution viscosities comparedto the untreated polymers. The polymers resulting from our inventionpossess the desirable processing properties so necessary for conjugateddiene polymers used in the manufacture of reinforced polystyrene and formaking rubber goods such as tires, conveyor belts and hoses.

Although the use of only organolithium initiators for synthesizingnon-terminated polymers has been shown in the experimental portion, thescope of the invention covers the use of other organoalkali metal andorganomagnesium initiators.

The microstructures of the polymers prepared from conjugated dienes maybe modified by employing polar compounds, known in the art, duringpolymerization. Some examples of polar compounds are: diglyme (dimethylether of diethylene glycol), tetrahydrofuran, triethylamine, andN,N,N',N'-tetramethylethylenediamine.

SUMMARY OF THE INVENTION

One aspect of the invention is a process for the preparation of blockcopolymers of vinyl-substituted aromatic compounds and conjugated dienesof broadened molecular weight distribution comprising:

(a) polymerizing a vinyl-substituted aromatic compound in the presenceof an initiator selected from organoalkali metal or organomagnesiuminitiators until the comsumption of the monomer is substantiallycomplete,

(b) adding one or more conjugated diene monomers and polymerizing untilsubstantially complete conversion of monomer(s) to polymer has takenplace, and

(c) reacting the resulting block copolymer from said steps (a) and (b)with a compound of the general formula ##STR2## wherein R₁ and R₂ aresame or different and are selected from a hydrocarbyl group containingfrom 1 to 12 carbon atoms and A, B and Y are oxygen or sulfur, X iscarbon or sulfur, with the stipulation that when X is sulfur Y must beoxygen, in an amount of from 0.2 to 3 moles of compound per mole of saidorganoalkali metal or organomagnesium initiators.

Another aspect of the invention is a process for the preparation ofconjugated diene polymers of broadened molecular weight distribution andnegligible cold flow comprising:

(a) polymerizing a conjugated diene or a mixture of conjugated dienes inthe presence of an initiator selected from organoalkali ororganomagnesium initiators until substantially complete consumption ofmonomer(s) has taken place, and

(b) reacting the resulting polymer from said step (a) with a couplingagent of the general formula ##STR3## wherein R₁ and R₂ are the same ordifferent and are selected from a hydrocarbyl group containing from 1 to12 carbon atoms A, B and Y are oxygen or sulfur, X is carbon or sulfur,with the stipulation that when X is sulfur Y must be oxygen, in anamount of from 0.2 to 3 moles of the coupling agent per mole of saidorganoalkali metal or organomagnesium initiator.

Still another aspect of the invention is conjugated diene polymers ofbroadened molecular weight distribution and negligible cold flowprepared by

(a) polymerizing conjugated dienes in the presence of an initiatorselected from an organoalkali metal or organomagnesium initiators untilthe consumption of monomer is substantially complete,

(b) reacting the resulting polymer from said step (a) with a couplingagent of the general formula ##STR4## R₁ and R₂ are the same ordifferent and are selected from a hydrocarbyl group containing from 1 to12 carbon atoms, A, B and Y are oxygen or sulfur, X is carbon or sulfur,with the stipulation that when X is sulfur, Y must be oxygen, in anamount of from 0.2 to 3 moles of coupling agent per mole of saidorganoalkali metal or organomagnesium initiator.

Also included are block copolymers of vinyl-substituted aromaticcompounds and conjugated dienes of broadened molecular weightdistribution comprising:

(a) polymerizing vinyl-substituted aromatic compounds in the presence ofan initiator selected from organoalkali metal or organomagnesiuminitiators until the consumption of monomer is substantially complete,

(b) adding one or more conjugated diene monomers and polymerizing untilsubstantially complete conversion of monomer(s) to polymer has takenplace, and

(c) reacting the resulting block copolymer from said steps (a) and (b)with a coupling agent of the general formula ##STR5## R₁ and R₂ are thesame or different and are selected from a hydrocarbyl group containingfrom 1 to 12 carbon atoms and A, B and Y are oxygen or sulfur, X iscarbon or sulfur, with the stipulation that when X is sulfur, Y must beoxygen, in an amount of from 0.2 to 3 moles of coupling agent per moleof said organoalkali or organomagnesium initiator.

It is noteworthy that in our invention for the coupling of "living"block copolymers, the hydrocarbon portion of the "living" chain end isalways derived from a conjugated diene.

As is well known in the art, the weight ratio or vinyl-substitutedaromatic compound to conjugated diene(s) in the "living" block copolymercan be varied widely, for instance, 5:95 to 95:5.

The molecular weight of the non-terminated polymers can be controlled bya judicious selection of the amount of monomers consumed duringpolymerization and the amount of initiator. The number average molecularweights may vary in the range of 30,000 to 350,000, the preferred rangebeing 60,000 to 200,000.

DETAILED DESCRIPTION OF INVENTION

Generally, the polymers that can be treated by the process of thisinvention are the living polymers of conjugated dienes containing from 4to 12 carbon atoms, preferably 4 to 8 carbon atoms, such as1,3-butadiene, isoprene, 1,3-hexadiene, 1,3-heptadiene, 1,3-octadiene,piperylene, 2-ethyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene,2-methyl-1,3-octadiene, 2-methyl-1,3-pentadiene,4-methyl-1,3-pentadiene, 3-methyl-1,3-pentadiene,4-methyl-1,3-heptadiene, 2-phenyl-1,3-butadiene and the like. Mixturesof dienes may also be used. The conjugated dienes can be polymerizedalone or in mixtures with vinyl-substituted aromatic compounds to formhomopolymers, copolymers or block copolymers. Block copolymers can beformed by sequentially polymerizing a vinyl-substituted aromaticcompound with an organoalkali metal compound and then adding aconjugated diene compound to produce a block copolymer having a terminalcarbon-alkali metal bond which can be subsequently reacted with acoupling agent. Vinyl-substituted aromatic compounds containing 8 to 16carbon atoms, preferably 8 to 12 carbon atoms can be polymerized withthe dienes. Examples of vinyl-substituted aromatic compounds arestyrene, α-methyl styrene, p-isopropylα-methylstyrene, vinyl toluene,3-methylstyrene, chlorostyrene, 4-cyclohexylstyrene, 4-p-tolylstyrene,1-vinylnaphthalene, 2-vinylnaphthalene and the like.

The polymers are prepared by contacting the monomer or monomers in aninert solvent with an organoalkali metal or organomagnesium compound.One of the preferred classes of these compounds can be represented bythe formula RLi wherein R is a hydrocarbon radical selected from thegroup consisting of aliphatic, cycloaliphatic, and aromatic radicalscontaining from 1 to 20 carbon atoms. Examples of these initiators aremethyllithium, n-butyllithium, sec-butyllithium, tert-butyllithium,n-decyllithium, phenyllithium, cyclohexyllithium, p-tolyllithium,n-eicosyllithium, and the like. Another class of initiators is thedilithium initiators such as DiLi-1™ and DiLi-3™ (Trademarks of LithiumCorporation), 1,4-dilithio-1,1,4,4-tetraphenylbutane,1,4-dilithio-1,4-dimethyl-2-butene and the like. Examples of otherinitiators which are useful in this invention are: sodium naphthalene,sodium biphenyl, benzyl sodium, cumyl potassium, cumyl cesium and cumylrubidium. When employing organosodium, organopotassium, organocesium andorganorubidium initiators, it is preferable to use them in an ethersolvent such as tetrahydrofuran to avoid side reactions.

It has been found (U.S. Pat. No. 3,822,219) that dialkylmagnesiumcompounds in combination with organoalkali metal compounds inhydrocarbon solvents catalyze the polymerization of conjugated dienes topredictable molecular weights. Some examples are: n--C₄ H₉ MgC₂ H₅ --RMand (n--C₆ H₁₃)₂ Mg--RM, where M is an alkali metal such as lithium,sodium or potassium, and R is an alkyl or aryl group.

The amount of initiator used varies, depending upon the desiredmolecular weight of the end product. The polymers are normally preparedat a temperature in the range between -100° and +150° C., preferably-75° and +75° C. It is preferred to carry out the polymerization in thepresence of a suitable inert solvent, for instance a hydrocarbon diluentsuch as benzene, cyclohexane, cyclopentane, n-pentane, hexane, heptane,octane, isooctane, and isopentane.

For environmental reasons, it is preferred that benzene be avoided(limitations on exposure to benzene vapors imposed by the Occupational,Safety and Health Administration). Aliphatic and cycloaliphatic solventsare preferred.

The microstructures of the polymers prepared from conjugated dienes maybe modified by employing polar compounds, known in the art, duringpolymerization.

The general class of coupling agents are linear organic compoundsselected from the group of carbonates, thiocarbonates and sulfites ofthe general formula ##STR6## R₁ and R₂ may be the same or different andare selected from a hydrocarbyl group containing from 1 to 20 carbonatoms, and A, B and Y are oxygen or sulfur, X is carbon or sulfur, withthe stipulation that when X is sulfur, Y must be oxygen. Examples of thecoupling agents are: dimethyl carbonate, diethyl carbonate, di-n-propylcarbonate, diisopropyl carbonate, di-n-butyl carbonate, diisobutylcarbonate, di-n-octyl carbonate, dicyclohexyl carbonate, diphenylcarbonate, ethyl phenyl carbonate, di-p-tolyl carbonate, cyclohexylphenyl carbonate, cyclopentyl butyl carbonate, O,O-diethylthiocarbonate, O,S-diethyl thiocarbonate, S,S-diethyl dithiocarbonate,O,S-diethyl dithiocarbonate, diethyl trithiocarbonate, diphenyltrithiocarbonate, dimethyl sulfite, diphenyl sulfite, ethyl phenylsulfite, and mixtures thereof.

The amount of a coupling agent used may be expressed in relation to theamount of polymerization initiator used above the scavenger level, whichtheoretically corresponds to the number of live polymer ends present inthe solution. Generally, the molar ratios of a coupling agent tocarbon-metal bond, for example, carbon-lithium bond, useful in thisinvention are from 0.05:1 to 5:1 preferably 0.2:1 to 3:1.

The coupling agent may be used neat or dissolved in an inert solvent.The reaction with a coupling agent is normally carried out with thesolution containing non-terminated polymer. However, for convenience andother considerations, the solution may be further diluted with thesolvent used during polymerization or with another desirable inertsolvent.

The coupling reaction may be carried out under atmospheric,subatmospheric or supraatmospheric pressures. The reaction temperaturemay be varied over a wide range, for instance, from about -50° to about200° C. It has been found that a temperature of 0° to 100° C. isconvenient for carrying the coupling reaction.

Cold flow was measured by extruding the polymer through a 1/16 inchorifice under constant pressure at a temperature of 122° F. Afterallowing 10 minutes at 122° F. to reach steady state, the rate ofextrusion was measured by weighing the amount of polymer extruded in 30minutes and recording the values in milligrams per minute.

BEST MODE OF THE INVENTION

The practice of this invention is illustrated by reference to thefollowing examples which are intended to be representative rather thanrestrictive of its scope.

EXAMPLE I

To each of four 8-oz. bottles were charged under high purity nitrogen5.5 g (0.102 mole) 1,3-butadiene and 161.5 ml benzene. After spargingthe solution with nitrogen for two minutes, 0.0560 molessec-butyl-lithium initiator (0.30 molar solution in hexane solvent)above the scavenger level was added. The bottle was fitted with a screwcap having a Teflon liner. The polymerizations were allowed to proceedat 25° C. for 20 hours. The specified amount of diethyl carbonate wasinjected (Table I), and the reaction allowed to continue for 24 hours.The resulting polymer solutions were precipitated in five-times thevolume of methanol containing 0.1% 2,6-ditert-butyl-p-cresol stabilizer.The conversion and molecular weight data on the dried polymers are shownin Table I. These data demonstrate that the coupling of livingpolybutadiene with diethyl carbonate yields gel-free polymers havingsignificantly higher molecular weights than their percursor.

                                      TABLE I                                     __________________________________________________________________________           Molar                                                                         Ratio Polymer                                                                              Inherent                                                                            Increase in                                                (EtO).sub.2 CO/                                                                     Conversion,                                                                          Viscosity,.sup.a                                                                    Inherent                                                                             %  No..sub.b Av. Mol.                        Polymer No.                                                                          sec-BuLi                                                                            %      dl/g  Viscosity, %                                                                         Gel                                                                              Wt.,  --Mn                                __________________________________________________________________________    1      0 (Control)                                                                         87     0.86  --     0   68,600                                   2      0.5   91     1.52  77     0  117,000                                   3      1.0   86     1.38  60.5   0  --                                        4      2.0   86     1.34  56     0  --                                        __________________________________________________________________________     .sup.a 0.1 gram polymer in 100 ml. toluene, 30° C.                     .sup.b Membrane Osmometer, toluene solvent, 30° C.                

EXAMPLE II

To each of three one-quart bottles was charged under nitrogen a solutionof 39.0 g (0.720 mole) 1,3-butadiene in 740 ml of benzene. Aftersparging with nitrogen, 0.264 mmoles sec-butyllithium initiator abovethe scavenger level was added, and the polymerizations were allowed toproceed for 20 hours at 25° C. The specified amount of diethyl carbonatewas injected (Table II), and the reaction allowed to continue for 24hours. The polymers were isolated by methanol coagulation and dried asin Example 1. The conversion data and physical properties are given inTable II. These data show that coupling of living polybutadiene withdiethyl carbonate led to a significant increase in the followingproperties as compared to the uncoupled polymers: inherent viscosity,no. av. mol. wt., styrene solution viscosity, and Mooney viscosity.Furthermore, polybutadiene coupled with diethyl carbonate exhibitednegligible tendency to flow in contrast to the pronounced tendency toflow by the control polymer. The ability of diethyl carbonate couplingagent to increase Mooney viscosity by about 240-350 percent and styrenesolution viscosity by 70-170 percent with only about 40 percent increasein number average molecular weight is quite unexpected.

                                      TABLE II                                    __________________________________________________________________________         Molar                                 Mooney                                                                             Cold                               Ratio Polymer                                                                              Inherent                                                                            Increase in                                                                             No. Av.  Viscosity,                                                                         Flow                          Polymer                                                                            (EtO).sub.2 CO/                                                                     Conversion,                                                                          Viscosity,.sup.a                                                                    Inherent                                                                             %  Mol. Wt..sup.b,                                                                        ML.sub.4                                                                           Index,                        No.  sec-BuLi                                                                            %      dl/g  Viscosity, %                                                                         Gel                                                                              --Mn  SSV.sup.c                                                                        (212° F.)                                                                   mg/min                        __________________________________________________________________________    5    0 (Control)                                                                         96     1.74  --     0  135,000                                                                             49 18.5 26.6                          6    0.5   94.5   2.65  52     0  191,000                                                                             135                                                                              85   0.6                           7    1.0   97     2.41  38.5   0  190,000                                                                             85 63.5 0.3                           __________________________________________________________________________     .sup.a Same as in Table I.                                                    .sup.b Membrane osmometer, toluene solvent, 30° C.                     .sup.c Styrene solution viscosity. Relative viscosity of 5 wt % solution      of polymer in styrene monomer at 30° C. to that of styrene monomer                                                                              

EXAMPLE III

In a manner similar to that in Example I, 5.5 g. (0.102 mole)1,3-butadiene, 161.5 milliliters of benzene, and 0.10 moles DiLi-3(dilithium initiator from Lithium Corp. 0.5 molar solution inhexane/triethylamine solvent) above the scavenger level were added toeach of two 8-oz. bottles. After polymerization for 20 hours at 25° C.the specified amount of diethyl carbonate (Table III) was injected.After isolation and drying, the polymers exhibited the inherentviscosity data shown in Table III.

                  TABLE III                                                       ______________________________________                                              Molar                                                                         Ratio     Polymer  Inherent                                                                              Increase                                     Poly- (EtO).sub.2 CO/                                                                         Conver-  Viscosity,                                                                            In Inherent                                                                            %                                   mer   DiLi-3    sion, %  dl/g    Viscosity, %                                                                           Gel                                 ______________________________________                                        8     0 (Control)                                                                             91       1.46    --       0                                   9     2         90       2.59    76       0                                   ______________________________________                                    

EXAMPLE IV

In a manner similar to that in Example I, 7.3 g. (0.135 mole)1,3-butadiene, 139 milliliters benzene, and 0.075 millimolessec-butyllithium above the scavenger level were added to each of three8-oz. bottles. After polymerization for 24 hours at 25° C., thespecified amount of a coupling agent (Table IV) was added, and thereaction allowed to continue at 25° C. for 24 hours. The polymersexhibited the data shown in Table IV. These data show that severallinear carbonates are effective coupling agents for livingpolybutadiene.

                                      TABLE IV                                    __________________________________________________________________________              Molar                                                                         Ratio  Polymer                                                                              Inherent                                                                            Increase in                                          Coupling                                                                           Coupling                                                                             Conversion,                                                                          Viscosity.sup.a                                                                     Inherent                                                                             %                                        Polymer                                                                            Agent                                                                              Agent/BuLi                                                                           %      dl/g  Viscosity, %                                                                         Gel                                                                              --Mw.sup.b                                                                        --Mn.sup.b                                                                        H.I..sup.c                    __________________________________________________________________________    10   none 0 (control)                                                                          85     1.25  --     0  200,000                                                                           140,000                                                                           1.42                          11   Diethyl                                                                            0.333  89     1.75  40     0  280,000                                                                           192,000                                                                           1.46                               carbonate                                                                12   Diisobutyl                                                                         0.333  83     1.48  18     0                                             carbonate                                                                __________________________________________________________________________     .sup.a See Table I.                                                           .sup.b Weight average and number average molecular weights.                   .sup.c Heterogeneity Index, --Mw/--Mn                                    

EXAMPLE V

If a block copolymer is prepared by the sequential addition of firststyrene and then butadiene using sec-butyllithium and is then reactedwith diethyl carbonate coupling agent according to the procedure inExample 1, the resulting product would have significantly higherinherent viscosity than the uncoupled precursor block copolymer. Theweight ratio of styrene to butadiene can be varied between 5:95 to 95:5.Similarly, the molecular weight of the uncoupled block copolymer can bevaried widely by a judicious selection of the amount of monomersundergoing polymerization and the amount of organolithium catalyst.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

We claim:
 1. A process for the preparation of conjugated diene polymersof broadened molecular weight distribution and negligible cold flowcomprising:(a) polymerizing at least one diene in the presence of aninitiator selected from organoalkali metal or organomagnesium initiatorsuntil the consumption of monomer is substantially complete, (b) reactingthe resulting polymer from said step (a) with a coupling agent of thegeneral formula ##STR7## wherein, R₁ and R₂ are the same or differentand are selected from a hydrocarbyl group containing from 1 to 12 carbonatoms A, B and Y are oxygen or sulfur, X is carbon or sulfur, with thestipulation that when X is sulfur, Y must by oxygen, in an amount offrom 0.2 to 3 moles of coupling agent per mole of said organoalkalimetal or organomagnesium initiator.
 2. The process according to claim 1,wherein the conjugated diene is selected from the group consisting of1,3-butadiene, isoprene, piperylene, 2-ethyl-1,3-butadiene, and2,3-dimethyl-1,3-butadiene, and the polymerization initiator is anorganolithium or an organosodium compound.
 3. The process according toclaim 1, wherein in the first step (a) the initiator is an organolithiumcompound and in the second step (b) the coupling agent is selected fromthe group consisting of dimethyl carbonate, diethylcarbonate, diisobutylcarbonate, diphenyl carbonate, O,O-diethylthiocarbonate, O,S-diethylthiocarbonate, O,S-diethyl dithiocarbonate, S,S-diethyl dithiocarbonate,diethyltrithiocarbonate, diphenyltrithiocarbonate, dimethylsulfite, anddiphenylsulfite.
 4. The process according to claim 1, wherein theorganolithium initiator is selected from the group consisting ofn-butyllithium, sec-butyllithium, phenyllithium,1,4-dilithio-1,1,4,4-tetraphenylbutane,1,4-dilithio-1,4-dimethyl-2-butene, DiLi-1 and DiLi-3.
 5. The processaccording to claim 3, wherein the organolithium initiator is selectedfrom the group consisting of n-butyllithium, sec-butyllithium,phenyllithium, 1,4-dilithio-1,1,4,4-tetraphenylbutane,1,4-dilithio-1,4-dimethyl-2-butene, DiLi-1 and DiLi-3.
 6. The process ofclaim 1 wherein the polymerization initiator is an organolithiumcompound, the conjugated diene monomer is either butadiene or isoprene,and the coupling agent is diethyl carbonate.
 7. Conjugated dienepolymers of broadened molecular weight distribution and negligible coldflow prepared by(a) polymerizing at least one diene in the presence ofan initiator selected from organoalkali metal or organomagnesiuminitiators until the consumption of monomer is substantially complete,(b) reacting the resulting polymer from said step (a) with a couplingagent of the general formula ##STR8## wherein, R₁ and R₂ are the same ordifferent and are selected from a hydrocarbyl group containing from 1 to12 carbon atoms, A, B and Y are oxygen or sulfur, X is carbon or sulfur,with the stipulation that when X is sulfur, Y must be oxygen, in anamount of from 0.2 to 3 moles of coupling agent per mole of saidorganoalkali metal or organomagnesium initiator.
 8. Polymers accordingto claim 7, wherein the conjugated diene polymerized is selected fromthe group consisting of 1,3-butadiene, isoprene, piperylene,2-ethyl-1,3-butadiene, and 2,3-dimethyl-1,3-butadiene, and thepolymerization initiator is an organolithium or an organosodiumcompound.
 9. Polymers according to claim 7, wherein in the first step(a) the initiator is an organolithium compound and in the second step(b) the coupling agent is selected from the group consisting of dimethylcarbonate, diethylcarbonate, diisobutyl carbonate, diphenyl carbonate,O,O-diethyl thiocarbonate O,S-diethyl thiocarbonate, O,S-diethyldithiocarbonate, S,S-diethyl dithiocarbonate, diethyltrithiocarbonate,diphenyltrithiocarbonate, dimethylsulfite, and diphenylsulfite. 10.Polymers according to claim 7, wherein the polymerization initiator isan organolithium compound, the conjugated diene monomer is eitherbutadiene or isoprene, and the coupling agent is diethyl carbonate.